This invention relates to semiconductor lasers and, in particular, to lasers having a combination index guiding and gain guiding features.
In conventional semiconductor lasers, the beam emitted from a facet of the laser is usually focused to a small spot in order to meet the needs of various applications, such as optical or magneto-optic disk storage and retrieval and laser printing, by means of reimaging the laser near field pattern to the desired image plane. Semiconductor lasers possess different points of origin of divergence (also referred to as beam waste positions) as well as angles of divergence for radiation emitted from the laser facet in directions orthogonal to each other, i.e., in a vertical emission direction which is perpendicular to the p-n planar junction and in a lateral emission direction which is parallel to and along the p-n planar junction.
These points of origin of divergence also vary relative to different kinds of laser geometry. One kind of laser geometry is the refractive index guiding laser which depends on differences in the index of refraction of materials due to either a diffusion, e.g., Zn, along the plane of the p-n junction to create lateral waveguidance or to the structural geometry of the laser, e.g., nonplanar layers, substrate channels or mesas, or layer thickness variations etc. to guide the propagating radiation by a real refractive index waveguide. In these lasers, the refractive index guiding referred to is along the p-n junction plane, although it need not be precisely at the plane. (It is understood that perpendicular to the plane of the p-n junction the heterostructure layers also create a refractive index waveguide). Examples of these lasers are the channeled substrate planar laser, the channeled substrate nonplanar laser and the buried heterostructure laser. In an index guided laser, the near field pattern of the laser can be imaged into a diffraction limited spot at an image plane with no correction for astigmatism because the beam wastes in both the vertical and lateral directions lie substantially in the plane of the laser facet. These lasers usually emit a narrow wavelength spectrum and often single longitudinal mode operation is obtained.
The other kind of laser geometry is the gain guide laser. A gain guided laser depends upon current dependent differences in both the real and imaginary part of the index of refraction of the semiconductor material comprising the structural layers of the laser to guide the propagating radiation. The narrow current confinement region, or narrow stripe, as the case may be, serves several purposes. Although the threshold current density is somewhat increased in operation of these lasers versus index guided lasers, the total laser threshold current is greatly reduced relative to a wide stripe which adds to minimize internal heating and thereby permitting continuous operation. The current confinement geometry confines the optical wave laterally in the p-n junction where no change in the real part of the refractive index exists in the absence of injected charges. Thus, the high injected charge density and resulting high gain directly beneath the current confining region determines both the real and imaginary parts of the lateral refractive index profile. This lateral waveguiding is totally dependent on the injected charge distribution. As a result, the laser characteristics will depend upon the widths of the current confining region.
As current confinement region widths decrease, power output increases stably (with increasing current) in gain guided lasers and the lateral mode does not shift. This improvement, however, occurs at the expense of an increase in beam divergence along the p-n junction plane and greater beam astigmatism, which is present in all gain guided lasers. Astigmatism occurs because the wavelength curvature of the laser beam is greater for narrower current confinement widths. Since no such curvature occurs perpendicular to the p-n junction plane, the beam waste in the p-n junction plane and in the plane in the laser optical cavity perpendicular to the p-n junction plane are at different spatial positions. Radiation in the vertical emission direction of the laser has a point of origin of divergence or beam waste position well within the laser behind the facet. Because of this factor, the image plane of the beam in the lateral emission direction will not be in the same plane as the vertical emission direction to bring about astigmatism upon focusing to a common image plane requiring more sophisticated optical systems for collimation and refocusing.
In applications such as optical disk systems, printer systems or other such applications requiring focusing of the near field output of a semiconductor laser, it is decidedly advantageous to employ an index guided laser since these lasers do not exhibit the astigmatism of gain guided lasers thereby permitting easily accomplished focusing. However, index guided lasers normally exhibit single longitudinal mode operation when operated at power levels in excess of several milliwatts. In the previously mentioned laser applications, single longitudinal mode operation leads to excess noise resulting from the long coherence length of the laser radiation or longitudinal mode "hopping" caused by thermal variation of the laser versus time. On the other hand, gain guided lasers will provide multilongitudinal mode operation relieving these undesirable effects but requiring a sophisticated lens system to remove the undesirable astigmatism.